Aetiological role of common respiratory viruses in acute lower respiratory infections in children under five years: A systematic review and meta–analysis

Shi, Ting; McLean, Kenneth; Campbell, Harry; Nair, Harish

Firstly, the virus could be an "innocent bystander" which is more prevalent in patients with ALRI, but has no causal role. Such an effect may be observed due to immunocompromised status from the true causal infection, or nosocomial infections. Secondly, the virus may be a risk factor for ALRI development, but not itself the primary cause. It has been well established that viral infections predispose to subsequent bacterial infection, although the exact mechanisms are still debated [59, 60] . Indeed, influenza and RSV epidemics are commonly observed to precede those of bacterial pneumonia [61] [62] [63] . Thirdly, the virus may be necessary to cause ALRI, but is not sufficient to do so without the concurrent presence of one or more other causal factors. There are numerous risk factors that have been associated with ALRI, both host and environment [50] . Furthermore, there has been lethal synergism observed in viral-bacterial superinfections [64, 65] . These, singularly or in combination, may provide the opportunity for the respiratory virus to cause ALRI. Fourthly, the virus may be the direct and sole cause of ALRI, with causality yet to be con-firmed. Finally, the virus may be the joint cause of ALRI along with other concurrent viral respiratory infections.